Blog

Ask almost anyone what they need to do to lose a few pounds, and they’ll probably say: “Cut back on the carbs.” As a personal trainer, I’ve heard it hundreds of times.

While the low carb movement has waxed and waned in popularity since the Atkins revival of the late 90s and early 2000s, most people now assume that carbohydrates are inherently fattening.

Health-conscious diners order bun-less burgers, skip the baked potato side dish, and send the bread basket back to the kitchen. (Or don’t, and feel guilty about it.)

In the past few years, I bet you’ve heard (or thought) at least one of the following:

Carbs spike your blood sugar and insulin, which causes you to put on body fat.

Carbs, especially sugar and grains, cause inflammation.

Carbs are not an essential part of the diet like fat and protein.

​Seems simple and logical. Which is the problem.

These simplistic statements about “good foods” and “bad foods” ignore biological complexity and the bigger picture.

Let’s look closer.

Do carbs increase insulin levels?Yes, they do.

Does increased insulin after meals lead to fat gain?No.(Insulin’s actually a satiety hormone. In other words, it makes you feel full. So the idea that on its own it leads to fat gain doesn’t make sense.)

Are carbs less important than protein, fat, and the many micronutrients that contribute to our health?Well, if you’re talking about processed carbs, the answer is a resounding yes.But if you’re talking about whole, minimally processed carbs, that’s a different story.

Can a low-carb diet work to help people lose weight?Of course it can.

Is it because it is low in carbs?Maybe. Maybe not.

Can eating an appropriate amount of carbs actually help you look, feel and perform your best?Absolutely! And that's the point, you need enough carbs - not too little, not too much.

The problem with not eating carbsAs a weight loss strategy, cutting carbs (while reducing the total number of calories) clearly works pretty well for some people. If it didn’t, then Atkins would have never been popular in the first place.

Here’s the thing, though: Carb reduction costs us.

You see, most of us require some level of carbohydrates to function at our best over the long term.

Sure, we can cut carbs temporarily if we need to lose weight quickly. But for most of us, keeping carbs too low for too long can have disastrous consequences.

This is especially true for those of us who exercise.

If you’re sedentary, your carb needs are lower. So you might be able to get away with more restriction.

But if you exercise regularly with intensity, restricting your carb intake too drastically can lead to:

And, unfortunately: You probably don’t even lose that much weight in the long term.

If you’re interested in the details and some research, read on. If you just want to know what to do, skip to the end.

Decreased thyroidIn order to function properly and to maintain an appropriate metabolism, our body produces an important hormone called T3. T3 is the most active thyroid hormone and is incredibly important for blood glucose management and proper metabolic function.

Low T3 levels can lead to a condition called euthyroid sick syndrome, in which people are constantly cold and sluggish. (Imagine your metabolic “body motor” idling at a slower speed.)

A landmark study, known as the Vermont Study, found that T3 is very sensitive to calorie and carbohydrate intake.

When calories and carbs are too low, your T3 levels drop.

In addition, the Vermont Study found that another hormone, reverse T3 (rT3), is also sensitive to calorie and carbohydrate intake. Reverse T3, as the name implies, inhibits T3.

Getting enough carbs can lower reverse T3. Not eating enough carbs will increase it, thus blocking the important work of T3.

The Vermont Study is far from alone. Other research confirms that ketogenic (ultra-low carb) diets reduce T3 levels as rapidly as starvation.

Additional studies show that when calories are held constant (in this case at 2100 calories), reducing carbohydrates from 409 g to 202 g and then to 104 g significantly reduced serum T3 levels (from 91 to 86 to 69 ng/dL respectively).

Finally, French researchers examined four calorically equal diets (2800 calories in this case), lasting 1 week each. Two of these diets contained 250 grams of carbs, which is a fairly typical proportion. The low-carb diet included 71 grams of carbs, and the high-carb diet included 533 grams of carbs.

T3 levels were equal on the normal and high carb diets (ranging from 163.3 to 169.5 ng/100 mL). However, on the low carb diet they fell to 148.6 ng/100 mL on average. And of course, rT3 correspondingly rose on the low carb diet, but not on the standard or high carb diets.

Thyroid hormones are important for more than just weight loss; they also have profound effects on our overall health and energy levels.

You feel terrible, and eventually your training energy becomes zapped.

If you’re active, you need adequate energy and carb intakes for a healthy thyroid.

Cortisol up; testosterone downResearch consistently shows that people who exercise regularly need to eat enough carbs or their testosterone will fall while their cortisol levels rise. This is a sure-fire recipe for losing muscle and gaining fat.

Incidentally, it’s also a marker for excessive training stress.

In a study in Life Sciences, men who ate a high carbohydrate versus a low carbohydrate diet for 10 days had higher levels of testosterone and sex hormone binding globulin, and lower levels of cortisol.

A few years later, another study took this research a step further. This time the subjects included men and women who exercised regularly. And in addition to considering the effect of their diet on hormones, researchers put them through some performance tests.

Once again, when the subjects ate a low carb diet, their testosterone (and other anabolic hormones) went down, while their cortisol went up.

And, after following a low carb diet for just three days, only two of the six participants were able to complete the cycling test! Meanwhile, when following the higher carb diet for three days, all six participants were able to complete the test.

In 2010, researchers reconsidered the same question. This time in relation to intense exercise. In this particular study, subjects eating the low carb diet (where 30% of their calories came from carbs) saw a drop of 43% in their testosterone to cortisol ratio. Not good. Meanwhile, the control group (who got 60% of their calories from carbs) saw no change in their testosterone/ cortisol ratios.

Thus:

inadequate carbohydrate intake can decrease testosterone (which no one wants); and

increase cortisol (which no one wants); while

negatively affecting performance (which no one wants).

Carbohydrates and women’s hormonesWe now know that eating too low-carb for too long can cause significant disruptions to many hormones.

This seems especially true for women, whose bodies may be more sensitive than men’s to low energy or carbohydrate availability (perhaps because of the evolutionary importance of having enough body fat and nutrients to sustain a pregnancy).

While organs like our testicles or thyroid make hormones, Mission Control of our hormone production system is the central nervous system (CNS), i.e. the brain.

Our hypothalamus and pituitary glands, which sit in the brain, are extremely sensitive to things like energy availability and stress (which can include life stress and exercise stress).

The hypothalamus and pituitary work together with other glands such as the adrenal glands. This partnership is often known as the hypothalamic-pituitary-adrenal, or HPA.

Thus, when women don’t eat enough calories or carbohydrate or even when women eat enough calories but not enough carbohydrate, they face hypothalamic amenorrhea.

This means disrupted hormones and stopped or irregular periods because of the HPA’s response to perceived starvation and stress.

In hypothalamic amenorrhea, hormone levels drop, and the cascade is felt throughout the system. You end up with low levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, progesterone, and testosterone.

In addition, we’ve already seen that not eating enough carbohydrates tends to increase cortisol levels. When cortisol rises, it signals your HPA axis to further decrease pituitary activity. Not good.

Again, you’d assume that protein intake would determine muscle breakdown. And you might assume based on what you’ve heard that having higher insulin is always “bad”.

Look, you need a balance.

In fact, insulin is crucial for building muscle.

When you get enough carbs to meet your needs, you replenish muscle glycogen and create an anabolic (building-up) hormonal environment. You get strong and toned. That’s good.

Conversely, when you don’t eat enough carbohydrate, muscle glycogen is depleted and a catabolic (breaking-down) hormonal environment is created, which means more protein breakdown and less protein synthesis. This means slower muscle growth or even muscle loss.

Even if all of this is true, aren’t low carb diets better for fat loss?

And aren’t fat-adapted athletes performing just as well as athletes who eat a lot of carbs?

Low carbs are not better for fat lossThe logic seems so clear and appealing: High carbs lead to insulin which leads to fat storage. Low carbs keep insulin low, which should get you effortlessly lean while you enjoy chicken wings, salmon, eggs, and butter.

Indeed, many people who try low-carb dieting are initially pleased by an immediate weight loss… which is mostly water and glycogen. So, in the short term, it seems like low-carb diets are superior.

But does long-term evidence support low-carb dieting?

Research says no. Over the long haul, any differences between ultra low-carb and other diets with more moderate carb levels even out.

Protein: The hidden success factorMost studies that suggest low-carb diets are superior suffer from a common methodological flaw: They usually don’t match protein intake between groups. This means that the low carb group often ends up consuming significantly more protein.

We know that getting plenty of protein has many advantages:

protein has a higher thermic effect — our bodies have to “rev up” to digest it (you’ll know this if you’ve ever gotten the “meat sweats” after a big steak);

protein makes people feel fuller, longer; and

protein helps people retain lean mass.

In other words, the big “secret” might be a high protein diet rather than a low carb diet.

So let’s keep it balanced and look at a study where protein was matched. In this study, subjects who ate a moderate carb diet (40% calories from carbs) reported significantly better mood, and lost about the same amount of weight as those on a ketogenic ultra-low carb diet (5% calories from carbs).

Actually, the group who ate a moderate amount of carbs showed a small (though not statistically significant) tendency to lose more body fat as compared to those on a low carb diet (5.5 kg vs 3.4 kg in 6 weeks).

Both diets improved insulin sensitivity. However, the ketogenic diet also increased LDL cholesterol and inflammatory markers and subjects who were on it felt less energetic.

Thus, in this study:

moderate carb eaters felt better

moderate carb eaters lost about the same amount of weight, maybe even a little more

both types of eaters improved insulin sensitivity

the low carb dieters ended up with worse blood work and inflammation

Makes you wonder why low carb gets so much hype, doesn’t it?

Especially considering that a recent review of long-term low carb versus low fat diets, the largest of its kind so far, found that both low carb and low fat diets reduced people’s weight and improved their metabolic risk factors.

In this review, both diets had about the same weight loss, changes in waist circumference, and measurements of several metabolic risk factors (blood pressure, blood glucose, insulin).

Still, it would be great to understand more about what makes low carb diets “work” at all. One recent study asked: Do low carb diets work because they restrict carbs or because they tend to increase protein?

Over the course of one year, the researchers compared four different conditions:

normal protein, normal carbohydrate

normal protein, low carbohydrate

high protein, low carbohydrate

high protein, normal carbohydrate.

Interestingly, the two groups eating the high protein lost the most weight.

And the real kicker? Varying the levels of fats and carbs seemed to make no difference to body composition.

Who needs carbs? Who doesn’t?Like most things, carbohydrate requirements fall on a bell curve.

Most people do best with some carbs.About 70% of you will do really well with a moderate level of carbs.

A few people do best with high carbs.About 2.5% of the population who are ultra-endurance athletes, and a few other outliers will thrive when eating incredibly high amounts of carbs. (We’re talking ≥ 70% of their total calories).

A few people do best with low carbs.In fact, ketogenic diets are actually prescribed for people with epilepsy, as they seem to reduce their symptoms and cut down on seizure frequency. There is also preliminary evidence that ketogenic diets benefit other neurological disorders, such as Parkinson’s disease and Alzheimer’s disease.

Very sedentary people, as well as people who are extremely metabolically deregulated (e.g. metabolic syndrome, diabetes), may benefit from a lower-carb diet for a while as part of an overall transition towards more activity and healthier metabolism.

A unique specimen: The low-carb athleteYou might have been wondering about that remaining 2.5% of successful low-carbers.

While rare, these ultra-low-carb people do exist. Even in athlete trials, where the vast majority of participants do better on high carb diets versus low carb diets, you’ll almost always find a few who perform better on a low carb regime.

This study on competitive cyclists offers a perfect example. While the authors concluded that endurance wasn’t generally affected by a high-fat, low-carb intake — at least after athletes became adapted to it — individual responses to this diet did vary enormously.

Two of the five participants got tired sooner when eating low-carb (taking 48 and 51 minutes to conk out, respectively). But one participant actually got better by 84 minutes on the low carb diet.

The data are clear: Each athlete — each person — is unique when it comes to carbohydrate requirements.

While on average the performance of the cyclists did not vary whether they were eating high carb or high fat diets, there was one interesting difference, highlighted by the study authors in a review study done twenty-one years later.

After a week of adaptation to the low-carb diet, most cyclists felt that they could more or less perform normally… except for their sprint capacity, which never seemed to recover while restricting carbs.

If you are a high-performing athlete, this might be especially important to keep in mind. Even in extreme endurance sports, sprint capability can be vitally important. Especially as you’re nearing that finish line.

But before you get too carried away in the opposite direction and start carb loading, let’s remember this basic truth:Most of us are not elite athletes.

So while studies will show that on average athletes tend to perform better with higher carb intakes, this is not a universal rule. There is always individual variability.

What this means for youSometimes, we get so caught up in fad diets that we forget to look at the evidence. But fad diets are mostly bad.

For many years, we thought the secret to maintaining our weight was to eat lots of carbs and reduce our fat intake.

Just think of the old Food Guide Pyramid with grains at the bottom and oils at the top.

Low-fat, high-carb didn’t work for most of us. People felt deprived and hungry; they “cheated” with “fat-free”, high-sugar treats; and they ended up eating a lot of rice cakes.

Then the pendulum swung, people hopped on the low carb, high fat bandwagon, and it was party time with almond butter, bacon, and heavy cream.

Unfortunately for most of us, low carb doesn’t work so well, either.

Strict diets are not the answerIf your eating plan isn’t working for you, it’s tempting to make it more restrictive. You might assume that if you aren’t losing fat going fairly low-carb, you should go full ketogenic (ultra low carb).

But more restriction almost never works.

Don’t take your nutrition to extremes, unless you have extreme goals.

Strategic moderation, is the only sustainable method.

Most of us need some carbsMost of us will look, feel, and perform our best when we balance a reasonable amount of lean protein, quality carbs, and healthy fats.

Experiment & have funIf you're curious about balancing your blood sugar by dialling back the carbs just a little bit, great, give it a go, monitor your glucose levels, and see how you feel.